This invention relates to a sensor which can be used to measure dielectric properties of materials.
It is well known that by measuring dielectric properties of a sample as a function of temperature, valuable information can be gained concerning the physical and chemical properties of the sample.
Two techniques are commonly used to measure dielectric properties. For many years measurements have been made by placing a sample between parallel plate electrodes, applying an electrical signal in the form of an alternating voltage, to one of the electrodes (i.e. the excitation electrode) and measuring the electrical signal from the other electrode (i.e. the response electrode). The following equation is used: ##EQU1## C=capacitance e.sub.o =permittivity of free space (a constant)
e'=permittivity of sample (being measured) PA1 A=area of parallel plate response electrode PA1 d=distance between the excitation and response electrode plates
By measuring capacitance, the permittivity of the sample can be easily calculated if the area of the parallel plate electrode and the distance between the excitation and response electrodes are known. A device for making measurements in this manner is disclosed in copending application Ser. No. 07/206,092. This technique is primarily used to characterize bulk properties of a material in that the signal is monitored through the entire thickness of the material. This technique has several limitations. Often times thick samples are of interest to be analyzed. In the parallel plate technique, the signal to noise ratio decreases as a function of increasing distance between the electrode plates. Larger plates could be utilized to increase the area thereby increasing the signal. However there does exist a practical limitation. Many times the surface of a material is to be analyzed. In polymer molding, skin effects are of interest due to faster cooling of the material's surface than its interior. The chemical and mechanical properties of the surface of the material are more indicative of its end use properties than the bulk interior properties. Coatings on a material surface are also of interest in dielectric analysis. Paints, adhesives, and copolymers often require analysis. A parallel plate measurement would detect the properties of the coating and its associated substrate in a bulk fashion. It is impossible to analyze surface characteristics by parallel plate analysis.
An alternate technique was developed and is commonly known which addresses the limitations of the parallel plate measurement. An interdigitated combed electrode is commonly used for obtaining dielectric measurements on surfaces of materials and fluids. Probes of this type have been used for many years as moisture detection devices. U.S. Pat. No. 3,696,360 to Gajewski, discloses an interdigitates electrode for moisture sensing. In the past few years these interdigitated probe structures were adapted to measure dielectric properites of materials. See, Society for the Advancement of Material and Process Engineering Journal, Volume 19, No. 4, July/August, 1983.
In this technique a sample is placed on the electrode surface, an electrical signal is applied to one "finger" of the interdigitated fingers or combs of the electrode array, and the signal is measured at the other finger of the array. These two fingers are termed excitation and response electrodes respectively. In this fashion the signal only penetrates the surface of the material. The penetration depth is approximately equal to the distance separating the fingers in the interdigitated electrode array. This technique is ideal for monitoring the dielectric characteristics of fluids, curing systems, adhesives, and relatively low viscosity materials. This technique, however, has severe limitations when analyzing films, hard plastics, and pre-cured systems (i.e., relatively viscous materials).
Problems arise due to surface contact with these types of materials. Interdigitated surface electrodes are inherently quite sensitive to whatever material contacts the surface, including air. Air gaps can severely limit the ability to measure dielectric properties of a material effectively since air and vacuum have the lowest permittivity theoretically possible (e'.sub.vac =1.00000). Thus, air gaps on the electrode surface will significantly, depress the measurement of a material's permittivity. Air also induces noise in the capacitance measurement. Unfortunately all electrode materials used in the fabrication of these surface sensors have a finite height or thickness. Obviously, a hard material will bridge the electrode fingers of the sensor. This bridge traps air between the electrode surface and the material in the spaces between the electrode fingers. This causes the measured permittivity to be the average value of the material's permittivity and the permittivity of air. This effect can depress the correct measurement of permittivity by as much as 50%.
Primarily dielectric measurements are made as a function of temperature to assess the characteristics of a material. As these viscous materials are heated beyond the glass-transition region they begin to flow and displace the air between the electrode fingers. This results in a dramatic elevation in the permittivity measurement. It would seem a viable option prior to analysis to preheat materials so that they "flow out" to fill the air gaps on the electrode to address this problem. Unfortunately, most materials change their molecular structure upon heating and cooling. This change effects the accurate measurement of glass transition temperature, degree of crystallinity, degree of cure, and most importantly, permittivity. Unfortunately the amount of air present between the electrode surface and the material being analyzed changes as the material softens with temperature. If the amount of air in this space remained constant a calibration could be made and the measurement could be corrected.
A surface analysis dielectric sensor is needed to eliminate the above mentioned limitations of existing interdigitated pectinate probes for use with relatively viscous materials.